This invention relates to a support system in a top driven drilling unit. More particularly, the invention relates to a handling system for supporting a drill string during tripping sequence when drilling a well and supporting a casing string when lining a well.
Conventional rotary drilling requires the use of a rotary table, a motor mounted on or below the rig floor for rotating the table, and a kelly for rotationally connecting the table to the drill string. In recent years, these drilling units are being replaced by or retrofitted with top driven drilling units which rotate the drill string using a motor suspended from a traveling block within a standard derrick or mast, hereafter referred to as a derrick.
Hereafter, a top driven unit will be referred to as a power swivel. The drilling motor is connected to the drill string by a cylindrical stem or sub assembly extending downwardly within the derrick from the drill motor. Drilling is accomplished by the powered rotation of the drill string. A cutting tool or bit is placed at the bottom end of the drill string which, through the rotational energy supplied by the drill motor, cuts through the earth's formations and deepens a well. As the well is drilled, the bit becomes worn and periodically must be replaced. When replacement of the bit becomes necessary, a portion of the drill string corresponding in length to one or more sections of drill pipe must be removed from the well and pulled above the drill rig floor. This portion of the drill string is removed and stored on the rig. The drill string is again pulled from the well exposing the next section above the floor and is similarly removed. This sequence, usually referred to as tripping out, is continued until the entire drill string is removed from the well. The bit on the bottom pipe section is replaced and the drill string is reassembled; i.e. tripping in, by connecting all the pipe sections previously removed. The drill string may be 10,000 feet (3050 m) or longer weighing 300,000 pounds (136,000 kg) or more. For a typical top driven drilling unit, the load of the drill string is supported by a swivel bearing during the tripping operations. The stress, impact and increased number of loaded cycles upon the bearing when supporting the drill string during tripping causes additional wear and may result in premature failure of the swivel bearing. Another concern is catastrophic failure of the swivel bearing housing caused by cyclical fatigue. Depending on the number of pipe lengths removed from the drill string during each "trip", the swivel bearing would be loaded and unloaded over 100 times when removing a 10,000 foot (3050 m) drill string from the well.
Because of many factors including unstable strata formations through which a well is drilled, the well normally will be lined with well casing. Lowering of the casing into the well is delayed for as long as possible while drilling a well. This casing may weigh more than 1,000,000 pounds (454,000 kg). Accordingly, the power swivel is normally removed from the derrick and replaced with a conventional hook assembly when casing is ready for running into the well.
Although they have a large operating cost advantage over conventional rotary table drilling units, top driven drilling units have structural deficiencies not previously anticipated. Excessive wear to the swivel bearing can result in premature failure resulting in costly downtime to remove and repair the power swivel. There is also concern for human safety because of potential structural failure of the swivel body resulting from cyclical fatigue during tripping. A further concern is the inability of power swivels to handle running of casing liners into a well with a proper safety factor. I have designed a power swivel having an improved structural system which overcomes the above stated problems and concerns. My structural system isolates the swivel bearing from elevator loads during tripping or when running casing liner. The load passes along a load path which bypasses the swivel bearing. The only time the swivel bearing of my power swivel is loaded occurs when the drill string is connected to the sub assembly during drilling or back-reaming. Accordingly, it is anticipated downtime and operation costs using the improved structural system of my power swivel will be reduced.